1. Field of the Invention
The present invention relates to a wavelength converting element in which an oscillation line at a predetermined wavelength is excited by making a laser beam incident to a predetermined laser medium as excited light and frequencies of the excited light and light on the oscillation line are mixed with each other by a nonlinear optical medium to emit light having a sum frequency. The present invention also relates to a system for recording and reproducing optical information and used in an optical pickup, etc.
2. Description of the Related Art
For example, a known wavelength converting element is shown in a document entitled "Pre-original collection 27a-ZL-4", p. 807 published in a fiftieth scientific lecture meeting held by the Society of Applied Physics of Japan in 1987.
This wavelength converting element has a problem that light emitting efficiency of a sum frequency is low.
Namely, for example, the above wavelength converting element is designed such that a laser resonator formed between mirrors functions as a resonator with respect to light having a wavelength of 1064 nm and excited in a Nd:YAG laser medium. In this case, the light having the wavelength of 1064 nm is amplified by the laser resonator and has large power within this laser resonator. However, light having a wavelength of 809 nm passes through a KTP crystal only once so that no power of this light is increased. Accordingly, no strong interaction between the lights having the wavelengths of 1064 nm and 809 nm can be obtained in the KTP crystal. Therefore, it is impossible to efficiently generate light having a sum frequency in the KTP crystal so that no light having a sum frequency can be efficiently emitted from the wavelength converting element.
A second higher harmonic wave generating element is called an SHG element in the following description. For example, this SHG element is disclosed in a literature "Journal of Optical Society of America B Vol. 3, No. 9, pp. 1175 to 1179 in 1986".
In this general SHG element, a semiconductor laser can directly emit LD light modulated at a high speed in the order of about several MHz by modulating a driving electric current of the semiconductor laser in the order of about several MHz at a high speed. However, a modulating frequency (or a modulating speed) of a Nd:YAG laser medium composed of a solid laser medium is limited and is of the order of about several KHz by its fluorescence lifetime of several hundred .mu. seconds. Accordingly, no solid laser such as Nd:YAG is modulated at a high speed in the order of several MHz even when the LD light modulated at a high speed in the order of several MHz is incident to the Nd:YAG laser medium from the semiconductor laser. SHG light finally emitted from a Nd:YAG laser resonator is not changed to a light beam modulated at a high speed in the order of about several MHz, but is changed to a light beam modulated in the order of about several KHz.
Therefore, in the general SHG element, the diameter of a converged light spot can be reduced, but no SHG light modulated at a high speed can be emitted from the SHG element. Accordingly, when the SHG element is applied to an optical pickup, etc., no information can be written, erased and read at high speeds.